The present invention relates generally to the field of radio frequency tag technology. More specifically, the present invention relates to passive radio frequency tags which are capable of changing state in response to an external stimulus.
Radio frequency (RF) tag technology has conventionally been used for identifying objects in radio frequency identification (RF ID) systems. In an RF ID system, information is carried on a tag (transponder) which is typically attached to an object of interest. When the tag comes within a RF signal field generated by a reader (transceiver) the tag responds to the incident RF signal. Typically, the tag reflects the incident RF carrier back to the reader in a form modulated by the tag according to the information with which the tag has been previously programmed.
RF tags may be passive or active. Active tags are powered by a battery which is incorporated into the tag. Passive tags do not have batteries. They derive their power inductively or capacitively from the RF signal transmitted by the reader to interrogate the tag.
FIG. 1A illustrates a conventional passive RF tag. The tag 100 has two main components: a semiconductor chip (integrated circuit (IC)) 102 having interface circuits, logic, and memory (not shown); and an antenna 104. The interface circuits of the IC 102 portion of a passive RF tag typically include an analog and a digital circuit. The analog circuit detects and decodes the RF signal and provides power to the digital circuit using the RF field strength of the reader. The digital circuit implements an information protocol which has been previously programmed into the tag. RF tags generally also include a variety of other discrete components, such as capacitors, clocks, and interconnections between components, a substrate for mounting components, and an enclosure.
FIG. 1B depicts a block diagram providing additional structural information for a typical passive RF tag. The figure is not a schematic depiction of an RF tag, but is intended as an illustration of the main function elements of a typical tag and their interconnections to provide a basis for describing the actions that take place when a tag (transponder) enters the RF field of a reader (transceiver), in order to assist in the understanding of the operation of RF tags.
An RF signal from a transceiver is received by the tag's antenna 110 when the tag enters the reader's RF field. From the antenna 110, the signal is typically smoothed by a capacitor 111, and split into a portion that provides the power for the tag, and a portion that provides the data to be read by and responded to by the tag's programmed logic. The power portion of the signal goes into a rectifier 112 (AC to DC converter) and the emerging DC signal is smoothed by a capacitor 104. The data portion of the split signal is conveyed along a conductive line 116 to a data extractor 118 which demodulates the signal and extracts the digital binary command data for the logic processor 120. The logic processor 120 receives the command and carries out the command instructions, which typically involves reading data from the tag's memory 122. The data read from the memory 122 is then output to a modulator 124 which modulates the digital data into an analog signal. The signal is then conveyed to the antenna 110 and transmitted back to the transceiver. RF tags also typically include additional elements not illustrated in FIG. 1B or discussed above, such as encoders/decoders and clock extractors.
As noted above radio frequency (RF) tag technology, particularly passive RF tag technology, has conventionally been used for identifying objects in radio frequency identification (RF ID) systems. Thus the conventional application of RF tags has been in tracking objects of interest. When the tag comes within a RF signal field generated by a reader (transceiver) the tag responds to the transceiver's incident RF signal alerting the transceiver of its presence. A typical reader includes a computer processor which issues commands to a RF transmitter and receives commands from an RF receiver. The processor may also perform one or more functions based on the tag's presence in its RF field.
For example, RF tags are used by airlines to track passenger luggage. When a passenger checks a piece of luggage it is tagged with an RF ID tag programmed with an identifier for that piece of luggage. When the luggage tag comes within the RF signal field of one of many RF ID readers located throughout the luggage system, the tag may be interrogated by the reader and the location of the luggage may be reported to a central tracking system by the reader's processor. Similarly, RF tag technology is used in “card key” systems. A card key contains a RF ID tag identifying the holder as a person authorized to pass through a door or gate. When the card comes within the RF signal field of an RF ID reader located at a door or gate, the tag may be interrogated by the reader and the authorization of the cardholder to pass may be confirmed, the door or gate my be opened, and the cardholder's passage recorded by the reader's processor.
While conventional implementations of RF tag technology have been useful in such tracking applications, the role of RF tags in these applications is static. That is, once a passive RF tag is programmed with information, it is simply polled by a reader. The tag may be reprogrammed with different information, but at any given time the tag has just one information state. The present inventors believe that RF tag technology offers the potential for a whole array of unexplored applications based on dynamic RF tags, that is, RF tags that are capable of existing in more than one information state without reprogramming. Accordingly, there is a need for the development of such dynamic, interactive RF tag technology.